A Norwegian helicopter hunting an Alaskan Malamute lands at an American Antarctic research station. As the Americans run out, a thermite charge destroys the helicopter and kills the Norwegian. Chattering madly in Norwegian, the other rifleman keeps firing at the dog and grazes Bennings. The Norwegian is killed by Garry, the station commander.
Helicopter pilot R.J. MacReady and Dr. Copper fly to the Norwegian camp for answers, but find a burned ruin, with the body of a man who committed suicide and a large block of ice with a hollowed cavity. Outside the camp, they discover the burned remains of a mangled humanoid corpse with two faces.
They bring the corpse back for an autopsy, but Blair can only tell them the creature's body contains a normal set of internal organs. Clark kennels the Malamute with the rest of the station's sled dogs where it metamorphoses and attacks them. MacReady hears the fight and hits the fire alarm, calling for a flamethrower and Childs incinerates the creature.
When Blair's autopsy reveals that the creature can imitate other beings, he suspects that anyone could be replaced and begins withdrawing from the others. The investigation of the Norwegians' records leads them to a massive crater containing a flying saucer and a hole left by the block of ice they suspect the creature came from.
Blair calculates that the alien can assimilate Earth within three years if it reaches civilization. Fuchs tells MacReady that the creature's "dead" remains are still active on a cellular level, according to Blair's journal. Bennings is attacked by the creature's remains, but as he metamorphoses he is caught outside and burned before he can escape and they incinerate the rest of the remains.
Blair, who was conspicuously absent until he is seen running from the wrecked helicopter, has killed the sled dogs and is destroying the radio when the team corners him and locks him in the tool shed. Determined to find out who is infected, they discover the blood stores are sabotaged before they can do the blood tests Copper recommends, and begin to turn on each other.
MacReady takes charge and orders Fuchs to continue Blair's work, but he disappears when the power goes out. As a storm closes in, MacReady, Windows, and Nauls search outside and find Fuchs' burned body, but Nauls cuts MacReady loose from the tow line on the way back, assuming he has been assimilated when he sees a torn shirt with MacReady's name on it. As the team debates MacReady's fate, he breaks in and threatens to dynamite the station if they attack him. Just then, Norris suffers a heart attack.
When Copper attempts to revive Norris by defibrillation, his chest gapes and closes like a giant mouth and bites off Copper's arms, who quickly dies from the injuries. MacReady burns the creature then orders everyone tied up for a new test. Clark tries to stab MacReady, but MacReady shoots and kills him. MacReady explains his theory that every piece of the alien is an organism with its own survival instinct that will react defensively when threatened.
One by one he tests everyone's blood, and they are all human except Palmer, who begins to transform and attacks Windows, forcing MacReady to burn them both. Leaving Childs on guard, the others head out to test Blair, only to find that he has tunneled under the tool shed. They realize that Blair is an alien who has scavenged the equipment he appeared to destroy in order to build a small escape craft.
They see someone run outside just before the camp loses power, and MacReady speculates that the alien intends to freeze itself until a rescue team arrives in the spring. They decide to dynamite the complex hoping to destroy the alien, but Garry is killed and Nauls disappears as they finish setting the explosives underground.
Blair transforms into a larger monster and attacks, but MacReady dynamites the monster and the base explodes as he escapes. Stumbling through the burning ruins with a bottle of scotch, MacReady finds Childs, who claims he got lost in the storm while pursuing Blair. Exhausted and with virtually no hope of survival, they acknowledge the futility of their distrust, sharing the bottle as the camp burns.
Resources: Wikipedia.org, imdb.com